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Abstract

We report the first observation of single-shot soft x-ray laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate) - PMMA. Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7nm, the samples have been investigated by atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze single-shot imprints in PMMA. An intermediate regime of materials removal has been found, confirming model predictions. We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32nm. Short-time exposure imprints provide sufficient information about transverse beam profile in HOH’s tight focus whereas long-time exposed PMMA exhibits radiation-initiated surface hardening making the beam profile measurement infeasible.

Figures (4)

(a) A crater created by the focused beam of FLASH at 21.7nm distinguishing between ablation and desorption regimes. (b) Transverse cross-section fitted by function (4) assuming a Gaussian beam profile.

(a) A shallow crater created by a pulse of radiation from FLASH at 21.7nm. The fluence is slightly above the threshold distinguishing the ablation, intermediate, and desorption regimes of material removal (A – the ablation region, I – the intermediate region, D – the desorption region). (b) AFM 3D morphology of the crater. (c) AFM surface roughness measurement for particular interaction regimes.